Transceivers, which in general comprise transmitters and receivers, employed in wireless communication devices, e.g. modern cellular phones, are usually highly integrated with most of the transceiver functions integrated on a Radio Frequency Integrated Circuit (RFIC). Highly integrated RFIC reduces phone's Printed Circuit Board (PCB) area, complexity and power consumption, while lowering cost of components. In addition, cellular receivers used in high-end mobile phones and laptops need to operate at multiple frequency bands and the cellular receivers have to support several wireless standards such as Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access (WCDMA), and Long Term Evolution (LTE) etc.
As each reception frequency band usually needs its own pre-selection filter between an antenna and the RFIC, the number of receiver inputs of the RFIC is basically determined by the number of bands needed to be supported. In practice, state-of-the-art RFICs may have as many as 10 to 30 receiver inputs. Moreover, as differential signal processing is considered to be more insensitive and robust against common-mode disturbances and interferences, often differential inputs are employed for receiver RFICs. Naturally, a corresponding first stage of the RFIC receiver, usually a Low-Noise Amplifier (LNA), is also implemented as a differential-input, differential-output amplifier. Unfortunately, as each differential LNA needs two input package pins, the number of RFIC package pins consumed by the receiver inputs will increase largely assuming that a large number of frequency bands needs to be supported. For instance, with 20 differential receiver inputs, altogether 40 package pins for the receiver inputs are needed in the RFIC. In addition, routing 20 differential Radio Frequency (RF) traces on PCB between the RFIC and a Front-End Module (FEM) containing pre-selection filters etc. becomes very challenging. For this reason, it would be very beneficial to have an LNA with a single-ended input so as to lower the number of RFIC package pins needed for the receiver. In addition, this would simplify the PCB routing between the FEM and RFIC, and also lower the PCB area and footprint needed for the corresponding routing. On the other hand, due to electrical performance reasons it is very beneficial to implement a down-conversion mixer following the LNA in the receiver downstream as a double-balanced circuit, so the LNA needs to have a differential output. As a result, a single-ended-to-differential LNA is needed.
The single-ended-to-differential amplifier may be implemented by using a single-ended amplifier, i.e. an amplifier with single-ended input and output, followed by a passive or active balun circuit, which converts a single-ended output signal of the amplifier to a differential signal. Unfortunately, single-ended amplifiers are very sensitive to poorly modeled ground and supply parasitics, such as parasitic inductances, which may degrade amplifier gain, input matching, Noise Figure (NF) etc. and in some extreme cases may cause circuit oscillation. As very accurate modeling of ground and supply parasitics is needed for the single-ended amplifier design, there is also a risk of penalty in time-to-market due to a longer design cycle. Moreover, in a product containing the RFIC, customers or another subcontractors may design the PCB, therefore it would be beneficial to use LNAs that are less sensitive to PCB parasitics, e.g. supply and ground inductances. Finally, unavoidable ground and supply parasitic loops may also act as a victim loop for magnetic coupling of undesirable spurious signals.
Usually, a passive balun circuit is implemented as an inductive transformer. However, a passive balun circuit or transformer circuit used at the amplifier output has usually lower quality factor than a corresponding differential inductor, which leads to power consumption penalty. Moreover, active balun circuits degrade performance of a receiver by introducing noise and nonlinearity while also increasing power consumption of the receiver.
It is also possible to realize a single-ended-to-differential amplifier by employing a balun circuit followed by a differential amplifier, i.e. an amplifier with balanced or differential input and output. The balun circuit converts a single-ended input signal to a differential signal for the differential amplifier. A conventional balun circuit may be implemented either as an on- or off-chip inductive transformer. However, as the loss of the balun circuit is very critical regarding the receiver NF, the balun circuit is usually implemented as an off-chip component with high Quality factor (Q-factor) and low loss. Unfortunately, since each RFIC receiver input needs its own balun circuit and external balun circuits are almost as expensive as pre-selection filters, the solution is not attractive due to high cost and a large PCB area is consumed.
U.S. Pat. No. 6,366,171 discloses a single-ended-to-differential LNA which can be integrated on silicon, but in this technique, a compensation circuit is needed to improve the differential signal phase imbalance. In addition, the auxiliary branch needed to generate the differential output signal generates substantial noise and nonlinearity.
In U.S. Pat. No. 7,646,250 and CHOI, J. et al., A Low Noise and Low Power RF Front-End for 5.8-GHz DSRC Receiver in 0.13 um CMOS, Journal of Semiconductor Technology and Science, Vol. 11, No. 1, March, 2011, single-to-differential signal converters with similar topology which can provide well-balanced output currents in response to a single-ended input voltage are disclosed. The topology is shown in FIG. 1, where the single-to-differential converter comprises a first transistor M1 and a second transistor M2, each configured as a common-source amplifier. Further, a capacitive cross-coupled transistor pair M3 and M4 is coupled to outputs of the first and second transistors M1 and M2. ZL is an LC-resonator circuit coupled at the output of the converter. Unfortunately, since this circuit has capacitive or imaginary input impedance, its input impedance cannot be matched to a real impedance, such as 50Ω, even with off-chip matching networks. As a result, the single-ended-to-differential converter shown in FIG. 1 cannot be used as an LNA in a wireless receiver as shown in FIG. 2 and described below, in which the LNA input impedance needs to be matched to a characteristic impedance, usually 50Ω, of a band-pass filter preceding the LNA.
In the wireless receiver shown in FIG. 2, an RF filter, or band pass filter, is needed to perform pre-selection of a received RF band. Without the RF filter, the linearity requirements of the receiver would be overwhelming and impractical. On the other hand, if the terminating impedance of the RF filter differs significantly from the specified characteristic impedance, it will cause large ripple and loss in the pass-band of the RF filter and worsen the transition band of the RF filter. Such large losses need to be avoided because they can, for example, lead to penalties in receiver NF and sensitivity. As a result, it is very important that the LNA presents sufficiently accurate terminating impedance for the RF filter.